1. Field of the Invention
The present invention relates to an optical transceiver (TRX), and more particularly to an optical transceiver including a light injected transmitter and a passive optical network (PON) using the same.
2. Description of the Related Art
Increasing interest in a wavelength division multiplexed passive optical network (WDM-PON) as a next generation subscriber network for providing future broadband communication services resulted in efforts to produce WDM-PON economically. Since WDM-PON allocates a separated wavelength to each subscriber, it requires a plurality of light sources and a wavelength division multiplexer (WDM) for multiplexing/de-multiplexing plural wavelength channels. In the WDM-PON, it is important to economically array wavelengths between these light sources and the WDM in order to reduce maintenance costs. Several suggestions of employing distributed feedback laser arrays, high power light emitting diode arrays, and spectrum-sliced light sources as the light sources have been made. Recently, a suggestion has been made to use a light injection-type light sources that determine output wavelengths by having light injected from outside. The suggestion intends to exploit the fact that such light sources can be maintained easily. Some examples of the light injection-type light sources include Fabry-Perot laser diodes (FP-LDs) and reflective semiconductor optical amplifiers (R-SOAs). One benefit of using light injection-type light sources is that wavelength of the light from the light sources need not be adjusted, for their wavelengths are determined by injected light. Accordingly, it becomes unnecessary to array the wavelengths between the light sources and the WDM, and it becomes simple to achieve and maintain network operation. Meanwhile, the WDM-PON has advantages such as a great bandwidth, superior security, and protocol independence. However, the WDM-PON is not widely used due to the high price of optical transceivers and the transmission difficulty of broadcasting data.
FIG. 1 is a block diagram showing a structure of a typical WDM-PON 100. The WDM-PON 100 includes a central office (CO) 110, a remote node (RN) 160 connected to the CO 110 through feeder fiber (FF) 150, and a plurality of optical network units (ONU) 190-1 to 190-N connected to the RN 160 through a plurality of distribution fibers (DF) 180-1 to 180-N. The CO 110 transmits broadband light (BL) to the RN 160 by intensity-modulating the broadband light based on broadcasting data, and the RN 160 spectrum-divides the modulated broadband light. Each ONU 190-1 to 190-N power-divides corresponding spectrum-divided lights and outputs optical signals, some of which are employed as electrical signals and others as injected light. Herein, the spectrum division refers to the operation of dividing light to a plurality of wavelength components, and the power division refers to the operation of dividing light regardless of wavelengths. The optical signals have wavelengths λ where the first wavelength has a wavelength λ1 and the Nth optical signal has the Nth wavelength λN.
The CO 110 includes a broadband light source (BLS) 120, an intensity-modulator (I-MOD) 130, and a circulator (CIR) 140.
The broadband light source 120 outputs a non-polarized incoherent broadband light. The broadband light source 120 may include an erbium doped amplifier (EDFA) having an erbium doped fiber (EDF) for amplifying spontaneous emission light, a laser diode for outputting pumping light for pumping the erbium doped fiber, and a wavelength selective coupler (WSC) for providing the pumping light to the erbium doped fiber.
The intensity-modulator 130 modulates the intensity of the broadband light, received from the broadband light source 120, based on input broadcasting data and outputs the broadband light. Generally, the intensity-modulator 130 may include a Mach-Zehnder modulator.
The circulator 140 includes three ports. The first port is connected to the intensity-modulator 130, and the second port is connected to the feeder fiber 150. The circulator 140 outputs the broadband light received by the first port to the second port and outputs multiplexed optical signal received by the second port to the third port. The circulator 140 represents a device that outputs light received from a certain port to the following port (the first port→the second port, the second port→the third port). Light received by the third port is extinguished.
The RN 160 includes a wavelength division multiplexer (WDM) 170.
The WDM 170 includes a multiplexing port (MP) and a plurality of de-multiplexing ports (DP). The multiplexing port is connected with the feeder fiber 150, and each de-multiplexing port makes one-to-one connection with respective distribution fibers 180-1 to 180-N. Accordingly, the Nth de-multiplexing port is connected with the Nth distribution fiber. The WDM 170 spectrum-divides the modulated broadband light received by the multiplexing port and outputs the modulated broadband lights to respective de-multiplexing ports. Accordingly, the WDM 170 outputs Nth spectrum-divided light to the Nth de-multiplexing port. In addition, the WDM 170 wavelength division multiplexes a plurality of optical signals received by the respective de-multiplexing ports and outputs the optical signals to the multiplexing port.
Each ONU 190-1 to 190-N has one-to-one connection with the respective distribution fibers 180-1 to 180-N, and each ONU includes optical transceivers 192-1 to 192-N. Accordingly, the Nth ONU 190-N, a representative of all ONU, is connected with the Nth distribution fiber 180-N and includes the Nth optical transceiver 192-N.
The Nth optical transceiver 192-N includes an Nth beam splitter (BS) 194-N, an Nth optical receiver (RX) 196-N, and an Nth light injected transmitter (TX) 198-N.
The Nth beam splitter (BS) 194-N includes three ports, where the first port to the third ports are connected with the Nth distribution fiber 180-N, the Nth optical receiver (RX) 196-N, and the Nth light injected transmitter (TX) 198-N, respectively. The Nth beam splitter (BS) 194-N power-divides the Nth spectrum-divided light received by the first port to first and second power-divided lights. The first power-divided light is output to the second port, and the second power-divided light is output to the third port. Also, the Nth beam splitter 194-N outputs the Nth optical signal received by the third port to the first port. The Nth beam splitter 194-N may include a Y-branch wave-guide.
The Nth optical receiver 196-N is connected with the second port of the Nth beam splitter (BS) 194-N and detects an electrical signal related to the first power-divided light from the Nth beam splitter 194-N. The Nth optical receiver 196-N may include a photodiode.
The Nth light injected transmitter 198-N is connected with the third port of the Nth beam splitter 194-N. The transmitter 198-N outputs the Nth optical signal that is generated by the second power divided light from the Nth beam splitter 194-N and that is modulated based on non-broadcasting data. The Nth light injected transmitter 198-N may include a Fabry-Perot laser diode (FP-LD) or a reflective semiconductor optical amplifier (R-SOA).
Hereinafter, a procedure for processing an optical signal in the WDM-PON 100 will be described. Broadband light generated from the broadband light source (BLS) 120 of the CO 110 is intensity-modulated in the intensity-modulator 130 based on broadcasting data, and the modulated broadband light is received by the WDM 170 of the RN 160 through the circulator 140 and the feeder fiber 150. The WDM 170 spectrum-divides the input broadband light to create a plurality of spectrum-divided lights. Each ONU 190-1 to 190-N power-divides corresponding spectrum-divided light. Each ONU then detects electrical signals based on a portion of the power-divided light and output, as injected light, respective optical signal using remaining portion of the power-divided light. Meanwhile, the WDM 170 multiplexes and outputs received a plurality of optical signals. Herein, the multiplexed optical signal is transmitted to the CO 110 through the feeder fiber 150.
In a typical WDM-PON 110, a portion of power of corresponding spectrum-divided light provided to each ONU is used for receiving broadcasting data. Accordingly a light injection efficiency is lowered. In addition, the intensity of injected light changes according to time due to the intensity-modulation based on broadcasting data. Such change in light intensity allows the output of the light injected transmitter to become unstable over time.